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ON
ON is used to connect the preposition "on" to certain time expressions.

	    		    +-----TY----+     
	  	       +-ON-+--TM-+ +-Xd+-Xc-+
	 	       |    |     | |   |    |
	I.p saw.v him on January 21 , 1990 ///// 

See "DT" for more discussion of time expressions.

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